Currently Cryptomator is still using WebDAV technology to provide the virtual hard drive you see in Windows Explorer. Technically WebDAV is a web protocol and MS Office programs think they need to be very smart handle communication via this web protocol using the Upload Center instead of letting the operating system do it.
Hence saving an Office document looks as if it would be uploaded. If you take a closer look at the “Speicherort”, you will see, that your .xlsx file is uploaded to http://cryptomator-vault/, which resolves to localhost. So it stored to your locally running Cryptomator instance.
Beginning with Cryptomator 1.4.0 we will provide an alternative to WebDAV, to be specific: Dokany. This will no longer cause the Upload Center to appear.